Where to override current_user helper method of devise gem

According to the module Devise::Controllers::Helpers, current_user (together with all other devise helpers) is added to ApplicationController, which means that you can override it in this way:

# in application_controller.rb

def devise_current_user
  @devise_current_user ||= warden.authenticate(scope: :user)
end

def current_user
  if params[:user_id].blank?
    devise_current_user
  else
    User.find(params[:user_id])
  end   
end